Iron-sulfur clusters are important cofactors required for the catalytic sites of various enzymes. Researchers have hypothesized that such clusters may have even been involved in the emergence of biological catalysis on early Earth. Such a role would have required spontaneous formation of these clusters from environmental sources, so identifying a plausible route has been a major goal in the field of prebiotic chemistry. Now, Sheref S. Mansy of the University of Trento and coworkers have identified one such plausible route. They report that UV light can drive the synthesis of [2Fe-2S] and [4Fe-4S] clusters from Fe2+ and organic thiols (Nat. Chem. 2017, DOI: 10.1038/nchem.2817). The researchers used glutathione, a cysteine-containing tripeptide, as a source of sulfur to make a model system. UV light liberates sulfide from the glutathione and oxidizes Fe2+ ions to Fe3+. The sulfide and ferric ions then combine to form the clusters. Because glutathione might not have existed on early Earth, the researchers also tested other cysteine-containing tripeptides and found that most of them could stabilize Fe-S clusters. In addition, they showed that formation of the clusters is compatible with fatty acid vesicles used to model early, simple cells.